Locations of existing CI platforms

jenkins.torproject.org (self-hosted; fed only by canonical repo on git.tpo)

oniongit.eu (self-hosted GitLab CI; resource-limited; only builds team member repos)

gitlab.com

travis-ci.org (fed by github.com)

Tradeoffs

GitLab is open source

GitHub is closed source

GitHub has a much larger user base. Network effects are important. (Also, we can't really expect external contributors to create an account on our self-hosted GitLab!)

Travis is easiest for any of us to fix when it breaks. Even external contributors can help if they have a fork! Just editing one file (.travis.yml is enough for most things). It also can build stuff pre-merge (including pull requests!).

Very few people have access to fix Jenkins when it breaks. Its summary output is also hard to read/navigate